fix(ai): fix serialization for Content with no parts field

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/Content.kt

@@ -90,7 +90,7 @@ constructor(public val role: String? = "user", public val parts: List<Part>) {
   @Serializable
   internal data class Internal(
     @EncodeDefault val role: String? = "user",
-    val parts: List<InternalPart>
+    val parts: List<InternalPart> = emptyList()
   ) {
     internal fun toPublic(): Content {
       val returnedParts =

firebase-ai/src/test/java/com/google/firebase/ai/DevAPIStreamingSnapshotTests.kt

@@ -88,6 +88,21 @@ internal class DevAPIStreamingSnapshotTests {
       }
     }
 
+  @Test
+  fun `streaming returned the last Content without parts`() =
+    goldenDevAPIStreamingFile("streaming-success-no-content-parts.txt") {
+      val responses = model.generateContentStream("prompt")
+
+      withTimeout(testTimeout) {
+        val responseList = responses.toList()
+        responseList.isEmpty() shouldBe false
+        responseList.last().candidates.first().apply {
+          finishReason shouldBe FinishReason.STOP
+          content.parts.isEmpty() shouldBe false
+        }
+      }
+    }
+
   @Test
   fun `stopped for recitation`() =
     goldenDevAPIStreamingFile("streaming-failure-recitation-no-content.txt") {